In high velocity injection mixing, two or more liquid components for forming a synthetic resin, for example a monomeric component and a catalyst, foaming, or blowing agent, are mixed thoroughly so that a uniform reaction occurs in a mold cavity or on a surface. The components must, however, be maintained separate until dispensing, since curing occurs quickly once mixed.
Foaming (mixing) of the reactants takes place by injecting the components under high pressure through separate opposed nozzle-containing inlets into a mixing chamber in a mixing head. The nozzle acts to accelerate the liquid component prior to injection into the mixing chamber to facilitate mixing. From the mixing chamber, the mixed reactants are dispensed through an outlet in the mixing head.
A mixing head of this type is used for injecting mixed reactants through port holes or gates in a closed mold cavity or for pouring into an open mold, which is thereafter closed. It may also be used for pouring or spraying onto open surfaces for making boards, sheet material, panels, or insulating layers, for example. In high velocity injection mixing, the mixing apparatus is often used for intermittent foaming, that is, the foam is dispensed in a shot-by-shot application. This is often the case in an assembly line setup, where the mixing head is placed over a mold, foam is applied to the mold and thereafter the mixing head is withdrawn. Between shots, the flow of the individual components to the mixing chamber is interrupted, stopping the foam output until the next mold is moved into place. The mixing chamber for this purpose usually is a cylindrical chamber. A hydraulically reciprocatable piston rod is disposed in the mixing chamber to move between a mixing position where components are injected through the opposed nozzles into the chamber and mixing takes place, and a blocking position where the piston rod blocks the nozzles and thus the flow of components into the mixing chamber.
Due to the stagnation of the flow in the mixing chamber, the mixed components tend quickly to clog the mixing head. Therefore, the mixing chamber must be cleaned between shots. However, stagnation of the flow of the individual components also occurs between shots. This tends to clog the delicate injection nozzle orifices and the flow passages, which can cause an improper mixture of components and thus a defective output, and eventually lead to blockage. Thus, frequent disassembly and cleaning of the mixing head is required.
In U.S. Pat. No. 3,706,515, a mixing head is shown and described for intermittent foaming in which, between shots, the mixing chamber is cleaned and the separate components are recirculated to prevent stagnation. A reciprocatable piston is movable between a mixing position in which the mixing chamber is open for injection of the separate components, and forward to a blocking position in which the mixing chamber is blocked. When in the blocking position, however, grooves formed in the piston connect the injection nozzles to recirculation lines. In addition, movement of the piston to the forward position scrapes the mixed reactants from the mixing cavity.
The device, however, possesses a number of shortcomings. When the piston is between the blocking and mixing positions in switching between recirculation and injection, the output nozzles are completely blocked. The piston thus must be moved quickly between positions and the recirculation grooves must have a long length extending to very near the front of the piston. This necessitates a long mixing head housing and requires a large, expensive hydraulic unit for reciprocating the piston.
The long length and speed of the piston movement renders the piston susceptible to freezing and scoring, especially if foreign particles, e.g. urea cyrstals, are present. This can be reduced to some extent if the mixing head parts are formed out of annealed and hardened high quality alloy steels, machined, ground, and finished to high precision. This, of course, is expensive.
Moreover, even if precision formed, a significant pressure fluctuation in the indivudual component pressure delivery systems occurs, which can cause the whole device to vibrate and shake, and can cause metering problems and lead/lag problems at the reinitiation of injection into the mixing chamber.
In certain applications, better mixing of the components occurs in a smaller mixing chamber, where the opposed nozzles are closely spaced. For effective recirculation, however, the grooves in the above-described device must be large enough in cross-section to offer little flow resistance. This dictates the size of the piston, which is then often too large for good mixing in these applications. Also, since the individual components are under high pressure, leakage from the lengthened piston grooves will occur without proper sealing. This requires precise dimensioning of the piston, cylinder and grooves along the entire length of the piston, as well as requiring seals along the piston to minimize intermixing of the reactive components. This is initially expensive. Also, the useful service life of the mixing head is short, since a minimum of normal wear causes fouling problems in the operation.